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First exomoon might have been spotted 4000 light years away

There’s a new target for Hubble

NASA

By Leah Crane

WE MIGHT have spotted a moon outside our solar system for the first time. And if the readings are accurate, it’s larger than any moon we’ve seen before.

David Kipping at Columbia University in New York and his colleagues have used the Kepler Space Telescope to search for moons around other worlds for years, without luck. “We’ve had candidates in the past and investigated them, and most of them have evaporated,” he says.

This latest possible moon was detected because of greater dips than we’d expect in the amount of starlight we can see as the planet passes in front of its star. Kipping and his team saw these additional dips, feasibly caused by a moon, over three of the planet’s orbits around its sun-sized star, Kepler-1625. That gave enough data for the team to say there’s only a roughly 1 in 16,000 likelihood of seeing such a signal by chance (arxiv.org/abs/1707.08563).

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“It is consistent with the signal that we might expect from a moon, but it might be consistent with other things as well,” says Kipping. The system is almost 4000 light years away and fairly faint, so more observations are needed to verify that the signal isn’t just a statistical blip.

The Hubble Space Telescope is more powerful than Kepler, so the group wants to point that at the star’s system in October, when the planet is expected to transit its star again, to get a clear view.

“We anticipate that the proposed measurements would be sufficient to confirm the first unambiguous detection of a moon beyond our Solar System,” the team writes in its request for time on the Hubble telescope.

“It’s such a faint star that it’d have to be a planet-sized moon for them to have seen it transit”

The group says the moon, if it exists, is probably the size of Neptune, and orbits a Jupiter-sized planet. Given what we know about how planets are born, it seems unlikely that the two formed together, but the large moon could have been captured by the planet later.

“If there really is something there, it’s such a faint star that it’d have to be a planet-sized moon for them to have seen it transit,” says David Waltham at Royal Holloway, University of London. Because of the many diverse moons in our solar system, most astronomers assume that there are lots of moons around more distant planets as well. “I think we’re pretty sure that they’re going to be there,” says Waltham. “It would be pretty odd that there are hundreds of moons in the solar system but none anywhere else.”

If Kipping and his team verify this detection, it would be the largest moon we’ve seen, as well as the first exomoon. And it would imply that there are more types of moon than the many we’ve already observed.

“It would be analogous to the first exoplanet detections, which defied our prejudices from the solar system,” says Duncan Forgan at the University of St Andrews, UK.

We’ll have to wait a few months to find out more. “It may prove to be nothing, or it may prove to be a really fabulous discovery,” says Waltham. “We won’t know until the Hubble data comes back.”

This article appeared in print under the headline “Glimpse of a huge moon in another solar system”